Field of the Invention
This invention relates to the field of brake adjusting tools and particularly to tools for checking the slack in the mechanism that applies braking force in air brakes of the type that use an S-cam.
The Air Brake Book published by Chilton's Commercial Carrier Journal at Chilton Way, Radnor Pa. 19809 describes a number of models of foundation air brakes on which the check tool of this invention can be used.
A typical foundation air brake used on each wheel, or wheel-pair, of large vehicles includes an air chamber rigidly mounted on a non-rotating portion of an axle structure near one of the wheels. A piston within the chamber is moved in one direction by compressed air controlled by a valve operated by the driver of the vehicle to apply the brakes by means of a push rod attached to the piston and connected by a clevis to a lever, called a slack adjuster, mounted on one end of a cam shaft. This movement of the lever transmits torque through the cam shaft to rotate an S-shaped cam, or S-cam, rigidly mounted on the other end of the shaft. The S-cam transmits the force through cam-follower rollers supported at the ends of brake shoes to force the brake shoes apart and the brake pads mounted on them against the brake drum to brake the vehicle.
The brake shoes are set to move a certain minimum distance before the pads touch the drums, and this distance constitutes slack in the movement of the slack adjuster lever. As the brake drum and the pads wear, the lever has to move farther to apply the brakes, which increases the range of slack movement. This freedom of motion must be limited so that it does not exceed a certain maximum amount, and for that reason the lever includes means for adjusting its angular setting on the cam shaft relative to the orientation of the S-cam. Thus, the slack adjuster is not directly affixed to the cam shaft but is rotatably mounted on a worm gear that has internal splines mounted on splines on the end of the cam shaft to prevent the gear from rotating relative to the shaft. The lever can rotate on the worm gear only to a limited extent within a range determined by the engagement of gear teeth on the outer surface of the gear with a worm mounted in a fixed location within the slack adjuster lever. The worm is rotatable about its own axis, which is perpendicular to the axis of the cam shaft, and it has a polygonal head, which is usually hexagonal but is square on some slack adjusters and is accessible from outside the slack adjuster to permit the worm to be rotated on its axis by a socket wrench. Excess slack is taken up by rotating the head to adjust the angle of the lever to the proper position to cause the S-cam to begin to apply pressure to the roller followers after the outer end of the push rod extending from the air chamber has moved only a short distance, typically between 1/2" and 3/4" from the position it occupies when the piston is not under pressure from compressed air.
Other slack adjusters are referred to as self-adjusting or automatic because they have sensors that detect the amount of slack and are connected to actuating means in the slack adjusting lever to rotate the worm to take up excess slack. However, even the automatic slack adjusters have provision for some manual adjustment.
The slack in each brake of a vehicle so equipped needs to be checked regularly to be sure that the brake shoes apply balanced pressure on the drums so that they can bring the vehicle to a halt without causing it to swerve or skid as it stops. In organizations that operate fleets of vehicles having air brakes, mechanics are required to inspect the brakes on a regular schedule, but in addition, the drivers are also supposed to check the slack adjusters at the beginning of each day's trip, if the slack adjusters can be reached. Such inspection requires getting under the vehicle far enough to be able to grasp the slack adjuster and to rotate it in the direction in which it is to pivot when the brakes are applied in normal operation of the vehicle. This can be an uncomfortable task in the wintertime, when the slack adjuster is likely to be icy cold, as well as in the summertime, when the vehicle and the pavement on which it sits are likely to be extremely hot, and drivers for truck fleets are frequently given gloves to protect their hands while manipulating the slack adjusters.